The preferred embodiments relate to electrical systems and methods and, more particularly, to improving circuit reliability by detecting and mitigating high voltage transient events at the circuit voltage supply.
During fast transient conduction in a passenger vehicle fitted with a 12V or 24V electric system, supply lines could have a spiking transient emission because of the inductance on electric wires. This voltage spikes could go up to 55V on top of the supply voltage. Therefore, the supply voltage total can spike up to 13V (typical supply voltage)+55V (peak of supply transient)=68V. Vehicle modules or systems also may include a charge pump, which as known in the art adds voltage to the nominal system supply voltage by switching voltage among one or more internal capacitors and to a final capacitance stage that can store a voltage greater than the input. In the event of a spike as described with a resultant voltage of 68V, therefore, then the output of the charge pump adds to the 68V spiked supply. For example, assume that the charge pump adds an additional 13V to the supply voltage; hence, when the supply spikes to 68V, then the additional 13V from the charge pump can bring the total potential to 81V (i.e., 68+13=81V). Thus, the output of the charge pump to ground can have the largest voltage difference in the system.
Excessive voltages from the combination of transients and a charge pump pose risk to other circuit structures and elements. For example, the charge pump output voltage may be used to drive the gate of an external switch device (e.g., MOSFET). Thus, the charge pump output voltage cannot be increased all the way up to the Vgs (i.e., gate-to-source voltage) limit of the switch, the device breakdown voltage, so as to not exceed the device breakdown voltage. A potential compromise, therefore, is to limit the charge pump output voltage to reduce the chance of breakdown, but such a limitation would likewise limit the turn-on resistance of the switch, too. As another example, internal PN-junctions must tolerate the charge pump output node potential, without breaking down. For example, any device connected to the high voltage node, such as the charge-pump output, will have a PN junction. For a fully isolated pMOS device, an isolation tank will be n-type doping area, which is connected to the highest voltage potential, and this will have a PN-junction to substrate. To make devices tolerable for such a high voltage, isolation tanks may be implemented. An increase in the protection of such devices, however, requires a corresponding increase in size and spacing, and, as a result, overall chip area would be significantly, and undesirably, increased.
Often device standards or specifications also must be satisfied in a system that will experience excessive voltages from the combination of transients and a charge pump. For example, the International Organization for Standardization (ISO) is a worldwide federation of national standards bodies (ISO member bodies), and in its ISO 7637-2, it specifies test methods and procedures to ensure the compatibility to conducted electrical transients of equipment installed on passenger cars and commercial vehicles fitted with 12V or 24V electrical systems. The possible maximum voltage peak during supply disturbance, as specified by ISO 7637-2:2011 5.6.2, and with the values given above would be 68V+13V=81V. One approach in this context, therefore, would be to select devices capable of withstanding the 81V peaks. In some manufacturing processes, however, such devices have isolated PN-junction ratings below or barely at these levels, so such options may be limited or even the most robust of the devices may still have questionable chances of surviving the peak voltages at, or slightly exceeding, its limit.
Given the preceding discussion, certain applications will have requirements that are not sufficiently addressed by the prior art. Thus, the present inventors seek to improve upon the prior art and address the considerations of such applications, as further detailed below.